Passive entry and passive starting systems are known for vehicles and allow a user to gain entry to a vehicle by simply operating a door handle and to remobilize passively and start an engine or other subsystem of the vehicle, e.g. by pressing a button. All this can be achieved by a user simply carrying a transponder about their person.
A system of this type might work, on detection of door handle operation, by sending a challenge to a remote transponder using a low frequency signal, e.g. 125 kHz. The transponder might then respond with an encrypted reply on a higher frequency, e.g. 433 MHz The low frequency (LF) signal may be sent from coils located near the front doors and boot and further coils may be installed in the interior of the vehicle so as to establish when the transponder is inside the vehicle to facilitate engine starting. This general type of passive entry and starting system is discussed in, for example, U.S. Pat. No. 4,973,958 and in EP 0783190.
It is a problem with some prior art security systems that a criminal can employ transmitter-receiver pairs with a two-way link between the vehicle and its owner. The criminal may succeed in gaining access to the car, even though the authorising transponder is not in his possession or even within range of the vehicle. One arrangement which provides protection against such so-called relay hackers is disclosed in our co-pending application GB 2332548.
There are other problems associated in particular with the “passive start” of a passive entry and passive enable/start arrangement. If, for example, detectors for passive starting rely on distance attenuation to determine whether a user is in the immediate locality of the drivers seat, it might prove difficult, due to the variability and shape of the magnetic fields, to guarantee completely reliable operation. For example a user carrying the transponder might be leaning against a driver's window while a child is standing on the driver's seat and this might cause the system to mistakenly determine that the conditions for enabling the starter switch had been satisfied.